Most industrial waters contain alkaline earth metal cations, such as calcium, barium, magnesium, etc., and several anions such as bicarbonate, carbonate, sulfate, phosphate, silicate, fluoride, etc. When combinations of these anions and cations are present in concentrations which exceed the solubility of their reaction products, precipitates form until these product solubility concentrations are no longer exceeded. For example, when the ionic product of calcium and carbonate exceeds the solubility of the calcium carbonate, a solid phase of calcium carbonate will form.
Solubility product concentrations are exceeded for various reasons, such as partial evaporation of the water phase, change in pH, pressure or temperature, and the introduction of additional ions which form insoluble compounds with the ions already present in the solution.
As these reaction products precipitate on surfaces of the water carrying system, they form scale or deposits. This accumulation prevents effective heat transfer, interferes with fluid flow, facilitates corrosive processes, and harbors bacteria. This scale is an expensive problem in many industrial water systems, causing delays and shutdowns for cleaning and removal.
Scale-forming salts can be prevented from precipitating by complexing the cations with chelating or sequestering agents so that the solubility of the reaction products is not exceeded. Generally, this requires stoichiometric amounts of chelating or sequestering agent with respect to the scale-forming cation, which amounts are not always desirable or economical.
More than 25 years ago it was discovered that certain inorganic polyphosphates would prevent such precipitation when added in amounts far less than the concentrations needed for sequestering or chelating. When a precipitation inhibitor is present in a potentially scale-forming system at a markedly lower concentration than that required for sequestering the scale forming cation, it is said to be present in a "threshold" amount. Threshold inhibition describes the phenomenon whereby a substoichiometric amount of a scale inhibitor can stabilize a solution from precipitation which solution can contain hundreds of thousands of parts of scale-forming ions. Threshold inhibition generally takes place under conditions where a few, i.e, 1 to 10 ppm, of a polymeric inhibitor will stabilize in solution from about 100 to several thousand ppm of a scale-forming mineral.
As already discussed above, whereas threshold inhibition occurs at substoichiometric ratios of inhibitor to scale-forming cation, sequestration requires a stoichiometric ratio of sequestrant to scale-forming cation to maintain that cation in solution. Generally, sequestering takes place at a weight ratio of threshold active compound to scale-forming cation components of greater than about ten to one, depending on the anion components in the water. Threshold inhibition, however, generally takes place in a weight ratio of threshold active compound to scale forming cation components of less than about 0.5 to 1.0. For example, a calcium sulfate solution containing 1820 ppm of calcium ions and 4440 ppm of sulfate ions is thermodynamically unstable. Unless a scale inhibitor is added, precipitation in such a system will take place within about one-half hour. To control precipitation of calcium sulfate from the supersaturated solution, the following two approaches are available:
(a) to complex or sequester calcium ions with a complexing agent such as ethylenediamine tetraacetic acid (EDTA) or nitrilotriacetic acid (NTA). Amount of each required to completely complex calcium ions would be stoichiometric, i.e, 1:1 ratio of Ca:EDTA or about 13300 ppm of EDTA to sequester 1820 ppm of calcium;
(b) on a threshold basis, one would need a substoichiometric amount of about 2 ppm of a polyacrylate to completely inhibit precipitation of calcium sulfate.
Therefore, on the basis of the above discussion, the tremendous difference between sequestration and threshold inhibition reflects the obvious advantages of the latter over the former.
In the past, chromate compounds and strong inorganic acids have been added to industrial water systems to reduce both corrosion of iron and scale formation. The chromate compounds have been used to reduce corrosion whereas strong inorganic acids have been used to reduce scale formation since scale formation is generally lower at acidic pH. Presently, however, the discharge of chromate compounds in the effluent is not permitted for the reason that the chromate compounds appear to be toxic and therefore, are deleterious to the environment.
To provide for the anticorrosion function of the chromate compounds of the past, polyphosphate and phosphate materials can and presently used as anticorrosion agents. Therefore, whereas in the past, emphasis has been on reducing scale formation of such scales as calcium carbonate and calcium sulfate, presently, the emphasis is on reducing formation of the phosphate scales such as calcium phosphate, zinc phosphate and magnesium phosphate.
The closest prior art known is the Booth et al U.S. Pat. No. 3,463,730 which relates to prevention of scale formation in water systems. More specifically, this patent relates to scale inhibition by the addition of up to 100 ppm of a hydrolyzed polyacrylamide to a water system containing insoluble salts, particularly carbonates and sulfates of metals such as calcium or other alkaline earth metals and/or iron, as well as particles of silt or silica. The polyacrylamide has about 10 to 50% unhydrolyzed amide groups and a molecular weight of about 1,000 to 8,000. It can be prepared in a number of different ways, including copolymerization of acrylic acid and acrylamide. By definition, the polyacrylamide described in the Booth et al patent has 50 to 90% of its amide groups hydrolyzed to acid or salt form.
Other pertinent prior art relating to phosphate inhibition includes U.S. Pat. Nos. 3,928,196, 4,029,577, 4,209,398, 4,253,968, 4,324,664 and 4,326,980. Generally, these patents disclose polymer inhibition compositions based on polymers of unsaturated carboxylic acids and other unsaturated monomers. Examples of unsaturated carboxylic acids include acrylic acid, methacrylic acid, and maleic acid. The other unsaturated monomers are diverse, but include hydroxyalkyl acrylates, allyl acetate, 2-acrylamido-2-methyl propanesulfonic acid, etc.